Dynamometer system

ABSTRACT

A system is provided for measuring the horsepower applied to the wheels of an automobile when the automobile is disposed on the rolls of a dynamometer. The system includes a bridge circuit formed by a plurality of strain gages. The strain gages are responsive to a torque imparted to the wheels of the automobile to produce an unbalance in the bridge in accordance with such torque. The system also includes means for producing a voltage related to the speed of rotation of the automobile wheels. This voltage is introduced to a first pair of opposite terminals in the bridge. A voltage representing the horsepower is then obtained from a second pair of output terminals in the bridge. One of the terminals in the second pair may be connected to a reference potential to eliminate the effects of any voltage common to the terminals in the first pair. Means may also be included in the bridge arrangement for controlling the speed of the vehicle and for compensating in the bridge for any unbalance produced by such control means.

United States Patent [191 Pelta Oct. 16, 1973 DYNAMOMETER SYSTEM EdmondR. Pelta, Pacific Palisades, Calif.

[73] Assignee: Autoscan, Inc., Los Angeles, Calif.

[22] Filed: Aug. 5, 1971 [21] Appl. No.: 169,308

[75] Inventor:

[52] US. Cl. 73/117 [51] Int. Cl. G011 5/13 [58] Field of Search 73/117,134, 135, 73/136 B [56] References Cited UNITEDSTATES PATENTS 2,746,2895/1956 "Cline .l 73/117 8/1951 Schaevitz 73/136 B Primary Examiner-JerryW. Myracle Attorney-Smyth, Roston & Pavitt [57] ABSTRACT A system isprovided for measuring the horsepower applied to the wheels of anautomobile when the automobile is disposed on the rolls of adynamometer. The system includes a bridge circuit formed by a pluralityof strain gages. The strain gages are responsive to a torque imparted tothe wheels of the automobile to produce an unbalance in the bridge inaccordance with such torque.

The system also includes means for producing a voltage related to thespeed of rotation of the automobile wheels. This voltage is introducedto a first pair of opposite terminals in the bridge. A voltagerepresenting the horsepower is then obtained from a second pair ofoutput terminals in the bridge.

One of the terminals in the second pair may be connected to a referencepotential to eliminate the effects of any voltage common to theterminals in the first pair. Means may also be included in the bridgearrangement for controlling the speed of the vehicle and forcompensating in the bridge for any unbalance produced by such controlmeans.

9 Claims, 2 Drawing Figures DYNAMOMETER SYSTEM This invention relates toa system for measuring hosepower in a dynamometer and particularlyrelates to a system which provides such a measurement by a simplecombination of electrical means and without any moving parts.

Measurements have been provided for a considerable number of years ofthe horsepower imparted to the wheels of an automobile which is disposedon the rolls of a dynamometer. In one type of apparatus, a first analogmeasurement is made of the speed of rotation of the wheels in theautomobile and a second analog measurement is made of the torqueimparted to the wheels of the automobile or the rolls of thedynamometer. The horsepower is then determined by multiplying the firstand second voltages on an analog basis. Such apparatus is not desirablebecause the analog multipliers are subject to drift so that theapparatus does not provide a reliable indication of horsepower.

In another type of apparatus for measuring horsepower, a tachometerproduces a voltage proportional to the speed of rotation of theautomobile wheels. This voltage is applied across a potentiometer havinga movable arm. The arm is moved along the potentiometer in accordancewith variations in the torque applied to the automobile wheels. Theresultant voltage on the movable arm indicates horsepower since itconstitutes in effect a multiplication of torque times speed. This typeof apparatus is complicated andunreliable, particularly since theinclusion of a movable arm causes errors to be produced in thepositioning of the arm and since the electrical contact between thepotentiometer and the arm eventually becomes worn.

This invention provides a system which indicates horsepower imparted toautomobile wheels or dynamometer rolls on a simple and reliable basis.The system is particularly reliable in view of its simplicity and inview of the fact it includes no moving parts. The system is alsoadvantageous in that it has a high speed of response and is able toindicate horsepower reliably and accurately regardless of the speed ofrotation of the automobile wheels.

The system includes a bridge circuit formed from a plurality ofresistance elements such as strain gages. The strain gages areresponsive to the torque imparted to the wheels of the automobile toproduce an unbalance in the bridge in accordance with such torque. Thesystem further includes means for producing a voltage related to thespeed of rotation of the automobile wheels. This voltage is introducedto a first pair of opposite terminals in the bridge..A voltagerepresenting the horsepower is then obtained from a second pair ofopposite terminals in the bridge.

One of the terminals in the second pair may be connected to a referencepotential such as ground to eliminate the effects of any voltage commonto the terminals in the first pair. By eliminating the effects of anysuch voltage, the voltage produced across the second pair of terminalsin the bridge provides an accurate and sensitive indication ofhorsepower. Means may also be included in the bridge arrangement forcontrolling the speed of the vehicle and for compensating in the bridgefor any unbalance produced by such control means.

In the drawings:

FIG. 1 is a schematic diagram of a dynamometer system for impartingtorque to the wheels of an automobile and for producing stresses orstrains in a plurality of strain gages in accordance with such torque;and

FIG. 2 is a circuit diagram of an electrical system constituting oneembodiment of the invention for indicating the horsepower imparted tothe wheels of the automobile.

In one embodiment of the invention, wheels 10 of an automobile 12 areadapted to be disposed on rolls 14 of a dynamometer generally indicatedat 16. An arm 18 is adapted to be mounted on the axle of at least one ofthe rolls 14, and a brake 20 is applied to at least one of the rolls 14to control the torque applied to the roll. Strain gages 22, 24, 26 and28 in a bridge generally indicated at 29 are mounted on the arm 18 at aparticular distance from the axis of the rolls. The strain gages 22, 24,26 and 28 are of conventional construction and are preferably matched inimpedance characteristics over the full range of stresses or strains andtemperature to a value less than 1 percent. The strain gages 22 and 24are disposed on one side of the arm 18 to become compressed by thetorque applied to the arm as a result of the brake force, and the straingages 26 and 28 are dis-' posed on the other side of the arm to becometensioned by the torque applied to the arm.

Although four strain gages are shown as being disposed on the arm 18, itwill be appreciated that a number of strain gages less than four may beprovided on the arm. For example, only the strain gage 22 may bedisposed on the arm and three other resistive members not subject tostrain may be used in place of the strain gages 24, 26 and 28.

The strain gages 22, 24, 26 and 28 are connected in the bridge circuit29 shown in FIG. 2. The strain gages 22 and 24 are disposed indiagonally opposite arms of the bridge and the strain gages 26 and 28are also disposed in diagonally opposite arms of the bridge. In this waythe strain gages 22 and 28 and the strain gages 24 and 26 arerespectively paired to define a first pair of opposite terminals 30 and31. Similarly, the strain gages 22 and 26 and the strain gages 24 and 28are re- .spectively paired to define a second pair of opposite terminals32 and 33.

Means are connected between the pair of opposite terminals 32 and 33 inthe bridge to indicate the horse power provided to the dynamometer. Suchmeans include an operational amplifier 34 having a pair of inputterminals and an output terminal. Resistances 35 and 36 are respectivelyconnected between the input terminals of the amplifier 34 and theopposite terminals 32 and 33 of the bridge. A resistance 37 is connectedbetween the output terminal of the amplifier and one of the inputterminals of the amplifer. A meter 38 is also connected to the outputterminal of the amplifier to indicate horsepower.

Means such as a tachometer 40 are adapted to be connected to the rollsof the dynamometer or to the wheels of the automobile to measure thespeed of the dynamometer and to produce a voltage having a magnitudedependent upon such speed. The tachometer 40 is indicated in brokenlines as including a rotary member 41 and as having an internalresistance 42. The tachometer 40 is connected in series with aresistance 44 between the opposite pair of terminals 30 and 31 in thebridge. An electrical arm formed by a meter 48 and resistances 50 and 52in series may be connected across the tachometer 40 to provide a visualindication of the speed of the dynamometer rolls.

Means may be included in the system constituting the invention forcontrolling the speed of rotation of the automobile wheels to anydesired value. Such means may include an operational amplifier 54 havinga pair of input terminals and an output terminal. A resistance 56 is inseries with the resistance 44 between the terminal 30 of the bridge andone input terminal of the operational amplifier 54. A resistance 58 isconnected between the other input terminal of the amplifier and theterminal 33 of the bridge. Feedback is provided from the output terminalof the amplifier through a resistance 60 to the first input terminal ofthe amplifier. The operational amplifier 54 has a high gain.

The output from the operational amplifier 54 is introduced through aresistance 64 to one input terminal of an operational amplifier 66having a second input terminal connected to a reference potential suchas ground. The first input terminal of the operational amplifier 66 alsoreceives a voltage through a resistance 68 from the adjustable arm of apotentiometer 70. The potentiometer 70 is in series with a resistance 72between a source of negativepotential such as approximately l5 volts andthe reference potential such as ground.

A rheostat 76 and a resistance 78 provide a feedback between the outputterminal of the operational amplifier 66 and the first input terminal ofthe amplifier. The output of the operational amplifier 66 is connectedto one terminal of a winding 80 in an electrical pressure transducerwhich controls the operation of an air amplifier. The air amplifier inturn controls the braking force applied to the dynamometer rolls 14. Theapplication of a braking force to the dynamometer rolls in accordancewith the operation of an air amplifier is disclosed in detail incopending application Ser. No. 856,535 filed by me on Sept. 10, 1969,for Test Apparatus and assigned of record to the assignee of record ofthis application.

The tachometer 40 provides a voltage having a magnitude substantiallyproportional to the speed of the dynamometer rolls 14. This voltage isintroduced to the pair of opposite terminals 30 and 31 in the bridge toapply a voltage across the bridge. When no braking force is applied tothe arm 18, all of the strain gages 22, 24, 26 and 28 are not understress so that the bridge is balanced. This causes voltages ofsubstantially equal magnitude to be produced at the pair of oppositeterminals 32 and 33 in the bridge so that no voltage is introduced tothe operational amplifier 34 and no indication is provided by the meter38.

As is well known, horsepower equals torque times angular speed times asuitable constant. The torque provided in the arm 18 causes a particularstress or strain such as compression to be applied to the strain gages22 and 24 and a stress such as tension to be applied to the strain gages26 and 28. The magnitude of such stress or strain is proportional to thetorque applied to the arm 18. The stresses or strains produced in thestrain gages 22, 24, 26 and 28 provide an unbalance in the bridge andthis unbalance produces between the pair of opposite terminals 32 and 33a voltage substantially proportional to the unbalance. Since the voltagebetween the terminals 32 and 33 is also dependent in part upon thevoltage produced by the tachometer 40, the voltage produced across theterminals 32 and 33 indicates horsepower by providing a multiplicationof torque times angular speed. This unbalance in the voltages betweenthe terminals 32 and 33 is indicated by the meter 38 which provides adirect indication of the horsepower.

Since the voltage between the terminals 32 and 33 represents theunbalance resulting from the stresses or strains in the strain gages 22,24, 26 and 28, this voltage is relatively low. For example, the voltagebetween the terminals 32 and 33 may be in the order of 30 millivoltswhen the voltage from the tachometer varies between 0 and 12 volts as aresult of variations in the speed of the dynamometer rolls 14. Since thevoltage between the pair of opposite terminals 32 and 33 is relativelylow, it is important to prevent common mode signals in the bridge fromoverwhelming the potential produced at the terminal 33. This common moderejection is produced by connecting the terminal 32 to a referencepotential such as ground so that the signals common to the terminals 30and 31 are cancelled. In this way, the bridge responds only to thevoltage produced between the terminals 30 and 31 to represent speed andthe unbalance produced in the bridge as a result of the stresses andstrains in the strain gages 22, 24, 26 and 28.

When the system constituting this invention is used only to measurehoresepower and not to provide speed control, the strain gages 22, 44,26 and 28 may be of the semiconductor type to provide for the productionof large changes in voltages as a result of the stresses and strainsimposed upon the strain gages. As a result, the horsepower can bemeasured directly across the terminals 32 and 33 without any need toamplify the voltages produced across these terminals. It will beappreciated, however, that strain gages of the semiconductor type mayalso be used in the bridge when the invention is used to provide speedcontrol.

The operational amplifiers 54 and 66 are included in the systemconstituting this invention when speed control is to be provided. Theoperational amplifiers such as the amplifier 54 buffer the voltagesintroduced to the input terminals so that any changes in the output fromthe amplifiers will not affect the inputs to the amplifiers.Furthermore, the operational amplifiers are provided so that thevoltages at the outputs of the amplifiers will be independent of anychanges in the output loads and particularly will be independent of anychanges resulting from changes in the positioning of the movable arm ofthe potentiometer 70.

The input impedance of the operational amplifier S4 is relatively low.Furthermore, since the operational amplifier has a high gain such as again of at least 100,000, the input voltage to the amplifier may berelatively low. This may be seen by dividing the output voltage by thegain to obtain the input voltage. Since the input impedance of theamplifier 54 is low and the input voltage to the amplifier is low, theresistances 56 and are connected effectively between the tachometer 40and the terminal 33. In order to balance the effect of these resistanceson the bridge, a resistance 81 and a rheostat 82 may be connected inseries between the tachometer 40 and the terminal 32 of the bridge. Thecombined values of the resistance 81 and the rheostat 82 may besubstantially equal to the combined values of the resistances 56 and 58.

The voltage produced at the output terminal of the amplifier 54represents the speed of the dynamometer rolls 14. This voltage isintroduced as a positive voltage to one input terminal of theoperational amplifier 66.

This input terminal also receives a negative voltage which is producedon the movable arm of the potentiometer 70. This negative voltagerepresents a desired speed of the dynamometer rolls 14. When the actualspeed of the dynamometer rolls 14 corresponds to the desired speed ofthe dynamometer rolls, the positive voltage on the output terminal ofthe amplifer 54 is substantially equal to the negative voltage on themovable arm of the potentiometer 70 so that no voltage is produced inthe operational amplifier 66. This causes the braking force applied tothe dynamometer rolls 14 to remain substantially constant. As will beappreciated, the amplifer 66 may be biased to maintain the braking forceat a value equal to the torque produced by the automobile so that thespeed of the automobile will not change.

When the output of the amplifier 54 is different in magnitude from thevoltage on the movable arm of the potentiometer 70, the differenceindicates that the braking force on the dynamometer rolls has to beadjusted so that the speed. of rotation of the automobile wheels will beable to reach the desired value. For example, when the operationalamplifier provides a voltage greater in magnitude than that on themovable arm of the potentiometer 70, the braking force should beincreased to decrease the speed of rotation of the automobile wheels sothat this speedwill be equal to the desired value. Similarly, when theoutput of the operational amplifier is a voltage less in magnitude thanthat on the movable arm of the potentiometer 70, the braking force onthe dynamometer wheels should be decreased to provide an increase in thespeed of rotation of the automobile wheels so that this speed will reachthe desired value.

As will be appreciated, the resistance values in the bridge change withvariations in the torque applied to the arm 18. If the effectiveresistance of the bridge can be expressed as a single resistance value,the value of this resistance will also change with variations in thetorque applied to the bridge. However, the changes in the value of thiseffective resistance will be relatively small, as may be seen by thefact that the voltage between the terminals 32 and 33 is only in therange of tens of millivolts. Since the effective resistance of thebridge changes only slightly with variations in applied torque, suchchanges in the effective resistance of the bridge will not affectappreciably the balance or the op eration of the amplifiers 54 and 66.

Although this application has been disclosed and illustrated withreference to particular applications, the principles involved aresusceptible of numerous other applications which will be apparent topersons skilled in the art. The invention is, therefore, to be limitedonly as indicated by the scope of the appended claims.

I claim: 1. In combination in a dynamometer for measuring the horsepowerprovided by an automobile having wheels which are disposed on rolls ofthe dynamometer,

first means for providing a torque to the dynamometer rolls, a

second means responsive to the speed of rotation of the wheels of theautomobile engine for producing a voltage related to such speed,

a strain gage having characteristics for providing variations inimpedance in accordance with the strain imparted to the strain gage,

an electrical bridge circuit having a plurality of electrical armsconnected in an arrangement defined by first and second pairs ofopposite terminals, the strain gage being connected in one of the armsof the bridge, the strain gage being operatively coupled to the firstmeans to receive strains in accordance with the torque provided by thefirst means and'to provide voltage variations in the bridge inaccordance with such strains and in representation of electricalunbalances in the bridge,

third means for introducing the voltage from the second means to thefirst pair of opposite terminals in the bridge, and fourth meansresponsive to the voltage across the second pair of opposite terminalsin the bridge for indicating the horsepower provided by the automobile,a particular one of the terminals in the second pair receiving a groundpotential to provide a rejection of the voltages common to the terminalsin the first pair. 2. The combination set forth in claim 1 wherein;fifth means are connected between a particular one of the terminals inthe first pair and the other one of the terminals in the second pair andare provided with a particular impedance for producing a control overthe-speed of rotation of the wheels of the automobile and wherein sixthmeans are connected between the particular one of the terminals in thefirst pair and the particular one of the terminals in the second pairfor compensating the bridge for the effect of the impedance provided bythe fifth means. 3. In combination in a dynamometer for measuring thehorsepower provided by an automobile having wheels which are disposed onrolls of the dynamometer,

an electrical bridge circuit formed from a plurality of strain gageseach having an electrical member having a variable impedance inaccordance with the strain imparted to the member, different pairs ofstrain gages being connected to each other in the bridge to providefirst and second pairs of opposite terminals, first means for impartinga braking action to the dynamometer rolls,

second means responsive to the braking action provided by the firstmeans for imparting a strain to the strain gages in accordance with suchbraking action,

third means responsive to the speed of rotation of the automobile wheelsfor producing a voltage related to such speed,

fourth means responsive to the voltage from the third means for applyingsuch voltage to the first pair of opposite terminals in the bridge, and

fifth means responsive to the voltage produced across the second pair ofopposite terminals in the bridge in accordance with any electricalunbalance in the bridge for indicating the horsepower provided by theautomobile, one of the terminals in the second pair receiving a groundpotential to provide a rejection of voltages common to the terminals inthe first pair.

4. In combination in a dynamometer for measuring the horsepower providedby an automobile having wheels which are disposed on rolls of thedynamometer,

an electrical bridge circuit formed from a plurality of strain gageseach having an electrical member having a variable impedance inaccordance with the strain imparted to the member, different pairs ofstrain gages being connected to each other in the bridge to providefirst and second pairs of opposite terminals,

first means for imparting a braking action to the dynamometer rolls,

second means responsive to the braking action provided by the firstmeans for imparting a strain to the strain gages in accordance with suchbraking action,

third means responsive to the speed of rotation of the automobile wheelsfor producing a voltage related to such speed,

fourth means responsive to the voltage from the third means for applyingsuch voltage to the first pair of opposite terminals in the bridge,

fifth means responsive to the voltage produced across the second pair ofopposite terminals in the bridge in accordance with any electricalunbalance in the bridge for indicating the horsepower provided by theautomobile,

a first operational amplifier and at least a first impedance beingconnected in series between first terminals in the first and secondpairs to provide an indication of the speed of rotation of the wheels ofthe automobile, and

at least a second impedance being connected between the first terminalin the first pair and the other terminal in the second pair and beingprovided with a value to compensate for the value of the firstimpedance.

5. The combination set forth in claim 4 wherein the second terminal inthe second pair is connected to a ground potential.

6. The combination set forth in claim 5 wherein adjustable means areprovided for providing an adjustable control voltage for a desired speedof rotation of the wheels of the automobile and wherein means includinga second operational amplifier are responsive to the voltage from thefirst operational amplifier and the adjustable control voltage forproducing a voltage for providing a control over the speed of rotationof the wheels of the automobile.

7. In combination in a dynamometer for measuring the horsepower providedby an automobile having wheels which are disposed on the dynamometer,

first means for producing a torque on the dynamometer rolls,

an electrical bridge circuit formed from a plurality of strain gages andhaving a plurality of different electrical arms connected in individualpairs to provide first and second pairs of opposite terminals and havingindividual ones of the gages in the different arms of the bridge, afirst pair of strain gages being mechanically coupled to the first meansto be placed in compression in accordance with the torque produced bythe first means and a second pair of strain gages being mechanicallycoupled to the first means to be placed in tension in accordance withthe torque produced by the first means to provide an electricalunbalance of the bridge in accordance with the compression and tensionimparted to the gages,

second means responsive to the rotation of the dynamometer rolls forproviding a voltage substantially proportional to the speed of suchrotation,

third means for applying the voltage from the second means to the firstpair of opposite terminals in the bridge circuit, and

fourth means responsive to the voltage produced across the second pairof opposite terminals in the bridge circuit for providing an indicationof the horsepower provided by the automobile, a particular one of theterminals in the second pair being connected to a ground potential-toprovide a rejection of the voltages common to the terminals in the firstpair.

8. In the combination set forth in claim 7,

fifth means connected across one of the arms of the bridge for providinga voltage for obtaining a control over the speed of the dynamometerrolls, and

sixth means connected across a second arm of the bridge for compensatingfor the effect on the bridge of the fifth means.

9. In the combination set forth in claim 8,

the fifth means being connected between a particular one of theterminals in the first pair and the other terminal in the second pairand including at least one resistance and an operational amplifier,

the sixth means being connected between the particular one of theterminals in the first pair and the particular terminal in the secondpair and having an impedance substantially equal to the impedance of thefifth means, and

seventh means for providing an adjustable voltage for comparison withthe voltage from the operational amplifier to provide a control over thespeed of the dynamometer rolls in accordance with such comparison.

1. In combination in a dynamometer for measuring the horsepower providedby an automobile having wheels which are disposed on rolls of thedynamometer, first means for providing a torque to the dynamometerrolls, second means responsive to the speed of rotation of the wheels ofthe automobile engine for producing a voltage related to such speed, astrain gage having characteristics for providing variations in impedancein accordance with the strain imparted to the strain gage, an electricalbridge circuit having a plurality of electrical arms connected in anarrangement defined by first and second pairs of opposite terminals, thestrain gage being connected in one of the arms of the bridge, the straingage being operatively coupled to the first means to receive strains inaccordance with the torque provided by the first means and to providevoltage variations in the bridge in accordance with such strains and inrepresentation of electrical unbalances in the bridge, third means forintroducing the voltage from the second means to the first pair ofopposite terminals in the bridge, and fourth means responsive to thevoltage across the second pair of opposite terminals in the bridge forindicating the horsepower provided by the automobile, a particular oneof the terminals in the second pair receiving a ground potential toprovide a rejection of the voltages common to the terminals in the firstpair.
 2. The combination set forth in claim 1 wherein; fifth means areconnected between a particular one of the terminals in the first pairand the other one of the terminals in the second pair and are providedwith a particular impedance for producing a control over the speed ofrotation of the wheels of the automobile and wherein sixth means areconnected between the particulAr one of the terminals in the first pairand the particular one of the terminals in the second pair forcompensating the bridge for the effect of the impedance provided by thefifth means.
 3. In combination in a dynamometer for measuring thehorsepower provided by an automobile having wheels which are disposed onrolls of the dynamometer, an electrical bridge circuit formed from aplurality of strain gages each having an electrical member having avariable impedance in accordance with the strain imparted to the member,different pairs of strain gages being connected to each other in thebridge to provide first and second pairs of opposite terminals, firstmeans for imparting a braking action to the dynamometer rolls, secondmeans responsive to the braking action provided by the first means forimparting a strain to the strain gages in accordance with such brakingaction, third means responsive to the speed of rotation of theautomobile wheels for producing a voltage related to such speed, fourthmeans responsive to the voltage from the third means for applying suchvoltage to the first pair of opposite terminals in the bridge, and fifthmeans responsive to the voltage produced across the second pair ofopposite terminals in the bridge in accordance with any electricalunbalance in the bridge for indicating the horsepower provided by theautomobile, one of the terminals in the second pair receiving a groundpotential to provide a rejection of voltages common to the terminals inthe first pair.
 4. In combination in a dynamometer for measuring thehorsepower provided by an automobile having wheels which are disposed onrolls of the dynamometer, an electrical bridge circuit formed from aplurality of strain gages each having an electrical member having avariable impedance in accordance with the strain imparted to the member,different pairs of strain gages being connected to each other in thebridge to provide first and second pairs of opposite terminals, firstmeans for imparting a braking action to the dynamometer rolls, secondmeans responsive to the braking action provided by the first means forimparting a strain to the strain gages in accordance with such brakingaction, third means responsive to the speed of rotation of theautomobile wheels for producing a voltage related to such speed, fourthmeans responsive to the voltage from the third means for applying suchvoltage to the first pair of opposite terminals in the bridge, fifthmeans responsive to the voltage produced across the second pair ofopposite terminals in the bridge in accordance with any electricalunbalance in the bridge for indicating the horsepower provided by theautomobile, a first operational amplifier and at least a first impedancebeing connected in series between first terminals in the first andsecond pairs to provide an indication of the speed of rotation of thewheels of the automobile, and at least a second impedance beingconnected between the first terminal in the first pair and the otherterminal in the second pair and being provided with a value tocompensate for the value of the first impedance.
 5. The combination setforth in claim 4 wherein the second terminal in the second pair isconnected to a ground potential.
 6. The combination set forth in claim 5wherein adjustable means are provided for providing an adjustablecontrol voltage for a desired speed of rotation of the wheels of theautomobile and wherein means including a second operational amplifierare responsive to the voltage from the first operational amplifier andthe adjustable control voltage for producing a voltage for providing acontrol over the speed of rotation of the wheels of the automobile. 7.In combination in a dynamometer for measuring the horsepower provided byan automobile having wheels which are disposed on the dynamometer, firstmeans for producing a torque on the dynamometer rolls, an electricalbridge circuit formed froM a plurality of strain gages and having aplurality of different electrical arms connected in individual pairs toprovide first and second pairs of opposite terminals and havingindividual ones of the gages in the different arms of the bridge, afirst pair of strain gages being mechanically coupled to the first meansto be placed in compression in accordance with the torque produced bythe first means and a second pair of strain gages being mechanicallycoupled to the first means to be placed in tension in accordance withthe torque produced by the first means to provide an electricalunbalance of the bridge in accordance with the compression and tensionimparted to the gages, second means responsive to the rotation of thedynamometer rolls for providing a voltage substantially proportional tothe speed of such rotation, third means for applying the voltage fromthe second means to the first pair of opposite terminals in the bridgecircuit, and fourth means responsive to the voltage produced across thesecond pair of opposite terminals in the bridge circuit for providing anindication of the horsepower provided by the automobile, a particularone of the terminals in the second pair being connected to a groundpotential to provide a rejection of the voltages common to the terminalsin the first pair.
 8. In the combination set forth in claim 7, fifthmeans connected across one of the arms of the bridge for providing avoltage for obtaining a control over the speed of the dynamometer rolls,and sixth means connected across a second arm of the bridge forcompensating for the effect on the bridge of the fifth means.
 9. In thecombination set forth in claim 8, the fifth means being connectedbetween a particular one of the terminals in the first pair and theother terminal in the second pair and including at least one resistanceand an operational amplifier, the sixth means being connected betweenthe particular one of the terminals in the first pair and the particularterminal in the second pair and having an impedance substantially equalto the impedance of the fifth means, and seventh means for providing anadjustable voltage for comparison with the voltage from the operationalamplifier to provide a control over the speed of the dynamometer rollsin accordance with such comparison.